Asymmetric glass-plastic laminates used as safety glass, impact resistant glass, penetration resistant glass, and the like are well known. Particularly, useful and well known among such laminates are those containing laminae of glass and polycarbonate. It is normal practice in the industry in constructing certain such laminates, to utilize glass or relatively hard solid resinous materials as the impact or shock receiving layers while utilizing the polycarbonate as the back or inner or downstream layer or that presented to the person or object being protected.
U.S. Pat. No. 3,666,614 discloses both asymmetric and symetric laminated assemblies for use as structural windshields in motor vehicles. These assemblies are glass-polycarbonate laminates, for example, two ply laminates having a forward facing lamina of glass and a back lamina of polycarbonate. Disclosed embodiments of this invention suffer from, among other deficiencies, a tendency of the back polycarbonate lamina to spall upon impact of an object with the front glass lamina and a tendency of the assymetric embodiments to warp.
U.S. Pat. No. 4,328,277 assigned to the same assignee as the present invention, discloses a bullet resistant laminate, preferably symetrical, having laminae of a solid resinous material such as polycarbonate or glass, adhesive interlayers of a compatible adhesive such as a polydiorganosiloxane-polycarbonate block copolymer, and a back mar resistant polycarbonate lamina of controlled thickness opposite the direction of impact or shock. The disclosed embodiments of this reference provide excellent penetration resistance for the weight and thickness. However, multi-ply asymmetric laminates constructed according to the mechanical variables and ply thicknesses of this disclosure in many cases contain potentially damaging residual stresses.
Thus, while laminates of glass and polycarbonate bonded with adhesive interlayers are well known, such asymmetric laminates having multiple laminae often contain excessive residual stresses at each interface and therefore are subject to warping and delamination. These phenomenon are particularly encouraged in circumstances where the laminate is at low temperature or where a temperature gradient is placed across the laminate.
Therefore, there being no teaching for multi-ply asymmetric laminates having reduced stress in the prior art, it is an object of the present invention to produce multi-ply asymmetric security laminates having less tendency to warp or delaminate.
It is another object of the present invention to provide an improved shock and penetration resistant multi-ply asymmetric laminate exhibiting reduced internal stress.
It is yet another object of the present invention to produce an asymmetric security laminate having a sufficient number of laminae to provide adequate shock and penetration resistance without the associated warping and delamination of residual stress.